The Forestry Commission has been using helicopters to spray pesticide on woodland near Pangbourne to kill oak processionary moth caterpillars

The number of young and adult blue tits was very low in the sprayed areas

A charity devoted to the conservation of insects says insecticide spraying in Pangbourne could have caused a dramatic drop in the number of blue tits.

Vanessa Amaral-Rogersm, campaigns officer with the invertebrates conservation trust Buglife, said the data came from surveys conducted by the Forestry Commission at sites in the Pangbourne area where it has been aerial spraying against the oak processionary moth.

The commission has twice sprayed two woodland areas including a Site of Special Scientific Interest in Tidmarsh using a helicopter in the last two years.

As part of the permission to carry out the spraying of the foreign moth, the Forestry Commission had to conduct surveys of wildlife in the spray sites and in comparable neighbouring areas.

Ms Amaral-Rogers said one finding showed there were just 25 green tortrix moth caterpillars in the sprayed area compared with 275 counted in the control area of unsprayed woodland nearby.

These caterpillars were important to the diet of baby blue tits.

Counts were made of great tits and blue tits in the two areas of woodland in Pangbourne and although there was little effect on the great tits, the number of young and adult blue tits was very low in the sprayed areas.

She said: “There were a large number of nests but very few adult and young birds which suggests that something happened to cause a drop in the number.”

The caterpillar of the oak processionary moth can be a human irritant because of its poisonous hairs

Ms Amaral-Rogers said one possible cause was the spraying for oak processionary moth.

She said the Forestry Commission’s approach to getting rid of the oak processionary moth was ‘a bit confusing’.

In Pangbourne, the number of caterpillar ‘nests’ dropped from 63 to three by searching for individual nests and ‘vacuuming’ them up without using pesticides.

She said: “It’s clear that this method was working perfectly well so it is difficult to see why spraying was necessary in Pangbourne.”

For the cost of one aerial spray, she said, the commission could have employed one person for a year to look for the nests and remove them without using pesticides.

However, she acknowledged that in London where there are widespread outbreaks of the oak processionary moth, this method would not be effective.

A Forestry Commission spokesman said: “We fully share Buglife’s concerns, which is why we have worked with Natural England and others to ensure that the treatment has the least possible impact on other wildlife.

“It is too soon to draw conclusions, and other factors, such as the weather, could also have influenced birds’ breeding success.

“Our five-year monitoring programme at Herridge’s Copse will give us a more accurate picture of the long-term effects.

“What is clear is that the consequences of not tackling oak processionary moth would be much more serious for Britain’s woodland wildlife, and for people and animals living close to oak trees.

“Manual removal of caterpillars and nests, which Buglife is advocating, can be useful when dealing with small numbers of trees in open situations such as fields, parks and gardens.

“However, it is not a practicable option in woodland, where the nests and caterpillars can be very difficult to find.

“So where any nests are found in woodland, there could be others which cannot be seen. One female moth can lay several hundred eggs, so just one nest producing dozens of moths can easily lead to a significant increase in the problem.

“We have been very successful in reducing the presence of this unpleasant pest in the Pangbourne area to the point where we might be close to eradicating it, and if we are to succeed we cannot afford to give the population a chance to recover.”

The caterpillar of the oak processionary moth can be a human irritant because of its poisonous hairs which may cause skin irritation and asthma.

The moth comes from central Europe and may have move north as a result of global warming.